Failed to create hard link - Operation not permitted on nfs share

We use unitrends uvb backup system for a vmware cluster. There a 4 storages (1 x OMV, 3 x Synology). The Openmediavault-Storage is formatet with xfs und mountet on the backupsystem by nfs. The Synologys are formatet with ext4 and also mounted by nfs to separate virual backup-appliances. The 3 appliances which use the synology storage are able to make hard links on the storage for a full backup. Only the one uvb appliance which is connected to the OMV Storage failed with the following message: "Failed to create hard link - Operation not permitted". There seems to be a permissions issue, but where?